The invention relates to a tire pressure sensor for motor vehicles.
The known tire pressure sensor for a vehicle includes a pressure switch which is fastened to the circumference of the wheel rim of a vehicle wheel. The pressure switch extends radially relative to the wheel axis and is actuated by the tire pressure. It is closed toward the tire by an electrically conductive diaphragm, which is oriented parallel to the running surface of the tire and is provided with an added mass for raising the switching threshold of the tire pressure sensor as a function of speed. This diaphragm is provided with a contact which is contacted by a contact surface at the central area of the diaphragm for closing an electrical circuit, when there is sufficient air pressure in the tire. The state of the electric circuit is to be monitored without the use of wires by a receiver in the vehicle, which is an evaluating circuit.
For high vehicle speeds of more than 160 km/h, tire and vehicle manufacturers prescribe increased tire pressures in order to ensure driving safety. In order to take this into account, the switching threshold of the pressure chamber must be adjusted to the highest required pressure. This leads to a loss of comfort at low speeds (DE-OS 32 43 845). If this is not taken into account and the tire pressure sensor is adjusted instead to the tire pressure prescribed in the lower speed range at full load, the required higher tire pressure is not monitored at high speeds, rather a nonexistent security is erroneously assumed because of the tire pressure monitoring device.
It is known from DE-OS 26 26 475 to provide the diaphragm of the reference pressure chamber of a tire pressure sensor with a solid piston whose front side is contacted by the inner tube of the tire to be monitored and which presses in the diaphragm against the pressure in the reference pressure chamber when there is sufficient air pressure in the tire until an electrical contact is made in the reference pressure chamber. It is also mentioned in this text that centrifugal forces at the piston of the diaphragm advantageously reduce the force exerted on the diaphragm by the tire pressure as the speed increases, so that the tire pressure must be increased at higher speeds into order to keep the switching contact closed. However, it is disadvantageous that this piston used as added mass for the diaphragm is covered by the inner tube of the tire, so that centrifugal forces acting on it are only partially effective, and in a completely indeterminate manner, at the reference pressure chamber for raising the switching threshold. Therefore, it is not possible with this solution to implement the increase in the switching threshold by a reproducible degree at a determined speed relative to standstill, since the speed-dependent raising of the switching threshold can not be determined beforehand in this known construction.